How to flip the mirror modifier? So I may edit the mirrored side
You can have the mirror modifier allow editing of both sides in edit mode by clicking the triangle icon in the modifier:

However, moving the mirrored side may be confusing, as it will always behave as if you are editing the corresponding selection on the non-mirrored side.
Flipping the model manually
There a few was to do this, but the fastest way I know of is pressing CtrlMX (or Y or Z, depending on your mirror axis) in object mode, making sure the pivot point is at the same spot as the mirror point.